A web-offset printing press includes an inking assembly for each color of ink used in the printing process. Each inking assembly includes an ink reservoir as well as a segmented blade disposed along the outer surface of an ink fountain roller. The amount of ink supplied to the roller train of the press and ultimately to a substrate such as paper is adjusted by changing the spacing between the edge of the blade segments and the outer surface of the ink fountain roller. The position of each blade segment relative to the ink fountain roller is independently adjustable by movement of an ink control device such as an adjusting screw, or ink key, to thereby control the amount of ink fed to a corresponding longitudinal strip or ink key zone of the substrate. The term "ink control device" is intended to include any device that controls the amount of ink fed to a corresponding longitudinal strip or zone of the substrate.
Typically, ink is also spread laterally from one longitudinal zone to adjacent zones due to the movement of vibrator rollers, which oscillate in a lateral direction relative to the substrate. The amount of ink on the ink fountain roller itself is also adjustable by changing the angle through which the ink fountain roller rotates each stroke. Typically, this occurs by adjusting a ratchet assembly, as is known in the art.
While the press is running, it is common for a press operator to continually monitor the printed output and to make appropriate ink key adjustments in order to achieve appropriate quality control of the color of the printed image. For example, if the color in a zone is too weak, the operator adjusts the corresponding ink key to allow more ink flow to that zone; if the color is too strong, the corresponding ink key is adjusted to decrease the ink flow. During operation of the printing press, further color adjustments may be necessary to compensate for changing press conditions, or to account for the personal preferences of the customer.
The above-described visual inspection techniques used in connection with ink key presetting and color control are inaccurate, expensive, and time-consuming. Further, since the required image colors are often halftones of ink combined with other ink colors, such techniques also require a high level of operator expertise.
Methods other than visual inspection of the printed image are also known for monitoring color quality once the press is running. Quality control of color printing processes can be achieved by measuring the optical density of a test target image. Optical density of various points of the test target image can be measured by using a densitometer or scanning densitometer either off-line or on-line of the web printing process. Typically, optical density measurements are performed by illuminating the test target image with a light source and measuring the intensity of the light reflected from the image. Optical density (D) is defined as: EQU D=-log.sub.10 (R) (E1)
where:
R is the reflectance, defined as the ratio of reflected light intensity to incident light intensity.
The test target image that is measured is often in the form of a color bar comprised of individual color patches. The color bar typically extends the width of the web. Typically, the patches include solid patches and halftone patches for each of the primary ink colors, as well as a few solid overprints. The color bar is often printed in the trim area of the web and may be utilized for registration as well as color monitoring purposes. Each solid patch has a target density that the color control system attempts to maintain. The inking level is increased or decreased to reach this target density. The halftone patches are also monitored (computing dot gain) to determine if the water balance is proper.
The newer short cutoff presses require the marks to be approximately 1.5 mm or less. A border of roughly 0.2 mm is required to accurately measure a color patch, leaving a width of only 1.1 mm or less to be used to calculate the reflectance or density value. Smaller color bars are more susceptible to the variation inherent to the printing process, and the validity of a reflectance or density reading on a narrow color bar is therefore questionable.
More importantly, however, is the fact that the color bars do not always indicate the colors of the work (the desired image to be printed). For example, a common practical problem encountered when running color on press is known as the "inline problem". This problem occurs when the inking requirements of two inline image portions (that is, two image portions in the same ink key zone) clash. For example, one image portion may consist of a red car, and the other may consist of a Caucasian face. In order to get the "cherry red" that the car requires, the magenta ink has to be run high. This unfortunately causes the face to appear too red. When a pressman runs into a situation such as this, the pressman attempts to find a magenta level that balances the facial tones against the red of the car.
Numerous systems have been developed for the control of color on a printing press that do not require the use of special test targets. For example, markless systems are described in U.S. Pat. Nos. 4,649,502, 4,660,159, 5,182,721, 5,224,421, 5,357,448, and 5,460,090.
On printing presses, there is a large variability in system gain from one job to the next. System gain refers to the amount of change in color due to a small change in ink feed amount. Control systems that can accurately predict the system gain and take into account the variability in system gain will converge faster than systems that have less accurate gain estimates.
Markless color control systems have been disclosed that describe empirical means for determining the system gain; for example, such systems are described in U.S. Pat. Nos. 4,660,159 and 5,182,721. Because the amount of coverage of a printing ink in a longitudinal strip has a large effect on the system gain for that ink, this must be repeated for each new job. This is a time-consuming process and one that produces additional waste products.
Alternatively, other systems disclose the use of a mathematical model to estimate the system gain. For example, U.S. Pat. Nos. 4,649,502 and 5,357,448 disclose the use of the Neugebauer equations, which relate dot area and reflectances of primaries to the reflectance of a halftone area. U.S. Pat. Nos. 4,649,502 and 4,660,159 make use of ink coverage in determining the system gain. U.S. Pat. No. 5,224,421 makes the assumption that the optical densities of inks are additive.
There are a number of aspects of the printing press that have been omitted from the models described in these patents. None of the models allow for a change in efficiency of inking with respect to coverage due to backflow of ink, lateral spread of ink due to the oscillatory vibrator rollers in the press, and the relationship between ink film thickness and reflectance. Exclusion of these factors from the model has an adverse effect on the accuracy of the determination of the system gain, and hence degrades system performance.
Markless systems are further limited in that no means are disclosed for precise alignment of the area of the printed material that is to be sampled. This is of most concern if measurements are to be made on a moving substrate, where timing and alignment of the sampling mechanism are difficult. Because of this, the sample points used by existing systems are forced to be larger than optimal, and preferably in an area of the work where the color is spatially constant.
A further aspect that is not considered by any of the markless color control systems disclosed in these patents is the visual effect of areas adjacent the sampling area on the appearance of a color within an image.
The systems described in U.S. Pat. Nos. 5,182,721, 5,224,421, and 5,357,448 require the collection of spectral data at a plurality of sampling points in the printed material. The use of spectral data is an additional expense and limits the practical number of sampling points that are available. U.S. Pat. No. 4,649,502 similarly has the additional expense of requiring an infrared channel.